The present invention relates to devices for reducing vibration in structural members, such as those of an aircraft fuselage.
Aircraft engines can induce significant vibration into the fuselage. In propeller powered planes, the propeller blades produce air pressure pulses which strike external surfaces thereby causing a time periodic vibration of the structure, at about 100 Hz for example, which vibration is transferred to other structural members of the airframe. Jet engines also produce vibration in supporting members. If left unchecked, the induced vibrations create objectionable noise in the aircraft cabin, and may result in serious fatigue of the airframe.
As a consequence, vibration absorbers are attached to structural members throughout the aircraft. For example, the Fokker 50 turbo-prop airplane carries approximately 150 frame-mounted absorbers. These devices typically are a simple spring-mass system in which a mass is attached to the airframe by a resilient member that acts as a spring. Elastomeric pads and metal cantilevers have been employed as the spring. The spring-mass system is fixedly tuned to resonate at the common frequency of vibration in the structural member of the airframe to which the absorber is attached and thus optimally absorbs the vibrational energy at that frequency. The absorber has a large mechanical impedance at resonance which is proportional to the quality factor Q, and the weight of the proof mass. Absorption (mechanical impedance) at other frequencies diminishes as a function of the deviation from the resonant frequency.
It should be recognized that vibration absorbers used in aircraft are exposed to large temperature variations during a typical flight. One drawback is that the tuning of these absorbers, and thus their effectiveness, changes as the spring material modulus varies with changes in temperature.